One of the things the pros know that you don't is what beats what - just by looking at the units and numbers. In an instant, they can tell whether they should press on or retreat.
The thing that newer players don't realize is that in Starcraft 2, you don't really just trade off units. That is, if you have a 15 food army and he has a 20 food army, he's not going to have 5 food left over after the fight. In fact, he'll probably have 10 or more, and then he'll come wipe you out before you can rebuild. Instead, if you retreat towards your base and get those few reinforcements, a losing battle becomes a winning one.
But how do you know? Experience. Whose experience? Anyone's. You can watch replays, other matches, and just start to know. You'll also pay more attention to this aspect of your game if you're not the one doing everything else, and just watch the battles, like a spectator.
Make this a good excuse to watch GomTV or Pro Replays.
PS - Thanks to freshmeatt for the correction.